The use of eddy current probes for inspecting metallic pipes and tubes for flaws and irregularities is well known in the art. By way of background, eddy current probes are devices which carry a transmit or energizing coil, energized by an alternating current signal, for inducing eddy currents in the tube wall proximate the coil and a receive or test coil for detecting the response of the tube wall to eddy currents. The induced eddy currents vary with the structure of the tube. Flaws and irregularities in the tube wall modify the eddy current. The output of the receive coil is monitored to obtain an indication of flaw and/or irregularity location. The probes are inserted into and made to travel along the tube. In some cases, the probe is rotated about an axis paralleling the tube axis so that the entire inner surface of the tube is monitored. In other cases, rather than rotating the probe, a plurality of transmit-receive coil combinations are provided to monitor the entire inner surface area of the tube. The latter devices tend to be more complex, expensive and not as effective. Common disadvantages of existing rotatable eddy current probes are that they have difficulty rotating in deformed or dirty tubes, the probes tend to wear out relatively quickly and they do not have or provide adequate space for the transmit-receive coils.
A known rotatable eddy current probe employs a flexible cable with a plurality of guides mounted on the cable at equal axial intervals therealong. The guides serve to keep the cable substantially concentrically disposed within the tube. One end of the cable is connected to a motor and slip-rings by a quick disconnect mechanism. A probe housing and slip-rings is secured to the cable between two guides. The housing comprises a main body portion which is non-rotatably secured to the cable. The main body portion houses a radially movable portion and a coil spring which urges the movable portion radially outwardly. The movable portion has a tube contact section and carries at least one eddy current probe. When the probe housing is inserted into a tube, the movable portion is urged inwardly by the tube wall against the action of the spring. The spring urges the movable portion and main body portion radially outwardly against diametrically opposed portion of the inner surface of the tube. While this arrangement operates reasonably well, there are a number of disadvantages. The housing requires at least two relatively complex parts--the main body and the movable portion. Thus, the manufacture and assembly of the probe body is relatively expensive. The movable portion, which carries the eddy current coils, is necessarily small and, therefore, offers only a small tube contact area which limits the size and number of coils and considerably increases the wear rate of the probe housing. Since the probe includes moving parts, the probe cannot be made of metallic parts, otherwise false eddy currents would be generated when there is relative movement of the parts. As a consequence, parts tend to wear out more quickly than they would if made of metal. The housing necessarily occupies a substantial portion of the tube. As a result, the operation of the probe will be subject to dirt and obstructions present in the tube. Dirt may become lodged in the housing and prevent proper movement of the movable portion.
There is a need for a probe body which is simpler to make and assemble, has a lower wear rate and, therefore a longer life, occupies less cross sectional area while at the same time providing a relatively large tube contact area so as to increase the sensitivity and life of the device.